Pt11 Vestibular System Flashcards
Describe vestibular dysfunction.
-head tilt, nystagmus, ataxia
Describe the vestibular system.
-bilateral receptor system located in inner ear
-divided into peripheral & central (clinical signs are diff)
sensory cells = vestibular hair cells
-has descending motor pathways = vestibulospinal tracts
Describe the inner ear (membranous labyrinth).
fluid filled with endolynph & surrounded by perilynph
-3 semicircular canals = kinetic labyrinth
-2 otolith organs = static labyrinth
-cochlea = audition
Describe the vestibular hair cells.
-large kinocilium
-tonically active cells at resting state
-constant firing rate when not stimulated
-stereocilia bent = AP freq increases
>bent other way = AP freq decreases
change in freq is how the brain knows which side we are moving
Describe the semi circular canals.
Function: detect rotation/angles of acceleration & deceleration of head
-anterior (superior) canal
-lateral canal
-posterior canal
Describe the vestibular hair cell canals.
-located in the crista ampullaris of semicircular canals & covered by gelatinous mass = cupula
-angular acceleration of head = cupula & stereocilia bend = excitation or inhibition of hair cells
-internal fluid moves in opp direction
-activate all 6 canals = precise indication of head movement in 3D
Describe the otolith organs.
-utricle (horizontal) & saccule (vertical)
-detect linear acceleration & deceleration & static tilt of head (gravitational force)
-hair cells located in macula of otoliths
-otolith overlies vestibular cells
What is the function of maculae?
-SPECIALIZED FOR SENSING LINEAR ACCELERATION, LIKE WHEN GRAVITY ACTS ON THE TILTING HEAD, OR IF THE HEAD STARTS MOVING IN A STRAIGHT LINE.
-THE DIFFERENCE IN INERTIA BETWEEN THE HAIR CELL STEREOCILIA AND THE OTOLITH IN WHICH THEY ARE EMBEDDED LEADS TO A FORCE THAT CAUSES THE STEREOCILIA TO BEND IN THE DIRECTION OF THAT LINEAR ACCELERATION.
Describe the relationship between posture & the vestibular system.
-when no lesion = equal extensor tone & balance (equilibrium)
-lesion on one side = lack of facilitation of extensor muscle on that side (imbalance)
>normal side will push body & head toward abnormal side (circling, leaning, rolling)
Describe extensor muscles.
-postural, antigravity muscles
-subconscious & involuntary
-long term contraction of lg group of muscles
-located close to spinal cord
Describe nystagmus.
-spontaneous nystagmus caused by abnormal, asymmetric AP input to brainstem from vestibular apparatus on the 2 sides of the head.
-imbalance in neural activity is interpreted by brainstem as rotation/movement of body
-nystagmus generated even if head & body are stationary
Describe the relationship between the position of the eye & the vestibular system.
-vestibular system responsible for maintaining position of eye relative to position of head in space
-control extraocular muscle of eye = vestibulocular reflex
>coordinates eye & head movements = head turns & eyes fixed on original field
>allows animal to interpret field of vision despite angular acceleration of head
>eyes slowly move opp of head rotation followed by fast flick in direction of head rotation = physiological nystagmus
What does the reflex eye movement pattern require?
-normal sensory input from semicircular ducts
-intact pathway in brainstem
-normal functional motor neurons
-extraocular muscles
Describe idiopathic vestibular disease.
-common in cats
-rolling & rapid nystagmus
-no sex/breed predilection
-~4 yrs
-resolve in 1 week
-diagnosed by excluding other known causes of peripheral vestibular disease like otitis
-some require fluid & antiemetic therapy
